Some of the motorcycles or automobiles known today use a drum brake apparatus as their braking apparatus. In the drum brake apparatus, brake shoes are pressed against the inner circumferential surface (frictional surface) of a brake drum rotating with a vehicle wheel to thereby control the rotation of the brake drum.
In general, the traditional brake drums are formed integrally, or in one piece, of cast iron in order to retain a necessary strength of the frictional surface; however, the traditional cast-iron brake drums are heavy in weight and thus would hinder desired weight reduction of the motorcycles or automobiles to which the brake drums are applied.
Thus, in some of the more sophisticated drum brake apparatus, a lightweight material, such as an aluminum alloy (hereinafter “Al alloy”), is used in a part of the brake drum with a view to reducing the weight of the brake drum. Namely, the frictional surface of the brake drum, which needs to have high resistance to abrasion, is formed of cast iron, and the remaining part of the brake drum is formed of an Al alloy or other lightweight material, so that the motorcycles or automobiles equipped with such a brake drum can be considerably lighter in overall weight and thereby achieves lowered fuel consumption.
However, in the field of the motorcycles or automobiles, there is still a great need for further weight reduction to achieve lower fuel consumption, and depending on the type of the brake drum used, the weight of the brake drum has to be lowered further in order to more appropriately meet such a need.